1. Technical Field of the Invention
The invention relates to the controlling of picture tubes used in monitors and television sets, more specifically to the linearization of their horizontal sweep.
2. Description of the Related Art
In picture tubes, the distance from the picture tube electron gun to various positions of the screen varies according to the deflection angle of the electron beam. Therefore, the position to which the electron beam points does not move at an even pace but more quickly by the edges of the screen and more slowly at the center of the screen if the electron beam is deflected by a steadily varying signal according to FIG. 1, in which case the patterns displayed on the screen will be distorted. In FIG. 1, the actual sweep period is the time between t.sub.1 and t.sub.0. The distortion is corrected using vertical and horizontal linearization, wherein the waveform of the signal controlling the electron beam is formed such that the angle of the electron beam changes in the border areas of the screen more rapidly than in the center, thus making the pixel formed by the electron beam to move evenly. Such a waveform is illustrated in FIG. 2.
The horizontal sweep current can be generated e.g. with the known basic construction illustrated in FIG. 3, disclosed in the Electronic Design, Jan. 4, 1973. In this circuit, during the second half of the sweep period, the transistor conducts and C2 acts as the energy source. At time to the deflection current through the deflection coil L1 reaches its negative peak and at the same time the transistor stops conducting and the retrace period begins. The energy stored in coil L2 builds up a sinusoidal voltage across the sweep capacitor C1. When this voltage returns to zero at time t.sub.1, Diode D1 begins to conduct and the sweep period begins again. The deflection coil L1 acts again as the energy source until time t.sub.2. At time t.sub.2 the current polarity reverses, the Diode stops conducting and the transistor starts conducting. Capacitor C2 is now the energy source until time t.sub.0.
The circuit described above generates in an ideal case a waveform according to FIG. 2. The differences between this waveform and the waveform of FIG. 1 are caused mainly by capacitor C2, this is why capacitor C2 is called an S-correction capacitor. Real components, however, have losses which make the curve asymmetric. The asymmetry can be removed e.g. by bringing in an external correction current to the deflection circuit via a transformer, for example. Disadvantages of this approach include the relative complexity and expansiveness of the circuits required to generate the correction current and feed it to the deflection circuit.
A generally used loss compensation method is to use a so-called linearization coil in addition to a deflection coil, connected in series with it, for example. The linearization coil is a coil the inductance of which depends on the current flowing through it. Such a coil can be made by using a ferromagnetic core in the coil and, in addition, a biasing magnet. FIG. 4 shows the optimum-shaped current-inductance curve to produce the correct sweep shape for such a linearization coil. This kind of solution according to a simple principle functions well when the line frequency is constant, as in television receivers. The waveform correction produced by the linearization circuit can then be tuned to correspond to the line frequency in use by moving the biasing magnet, which effectively moves the inductance curve in the direction of the current axis in FIG. 4.
In monitors, the situation is more complex because they can use many different line frequencies and therefore the inductance of the linearization coil must be changeable according to the line frequency. Such a solution can be implemented by using a FET transistor to connect a second coil in parallel with the linearization coil and using the FET transistor to direct part of the sweep current past the linearization coil. The disadvantage of this kind of solution is that it requires many additional coils and FET transistors, depending on the number of line frequencies used in the monitor. Generally, however, to save costs, only one additional coil and FET is used.
Nowadays, a more commonly used solution is to use a linearization coil including a control winding, wherein direct current is brought to the control winding to compensate for the biasing magnet field of the linearization coil in order to change the inductance at the ends of the linearization coil. The current through the control winding and thus the inductance of the linearization coil can be changed according to the line frequency in use. In this solution, the disadvantage is that the curved shape of the current-inductance curve is not retained, as shown in FIG. 5 where the curvature is shown flattened or substantially reduced in curvature, which causes nonlinearities in the monitor display.
In addition, the tuning of the linearization circuits must be very accurate. Variation of characteristics of new component lots, even within the normal tolerances, may affect the tuning so much that the produced circuit has to be separately tuned after manufacturing. As far as devices with fixed coils are concerned, one has to either reduce turns in the linearization coil or add extra coils in parallel with the linearization coil.